Lactic acid is widely used for food, pharmaceuticals and the like, and also widely applied to industrial uses as a monomer material for polylactic acid, which is a biodegradable plastic, so that its demand is increasing. Lactic acid is known to be produced by fermentation by microorganisms which convert carbohydrate-containing substrates represented by glucose into lactic acid.
To obtain lactic acid as a raw material for polylactic acid, a highly productive production method of lactic acid is required since the necessary amount of lactic acid is large. To enhance productivity of lactic acid, a high yield relative to sugar consumption in the microbial fermentation as well as a high production rate of lactic acid per unit time per unit volume are indispensable and, in WO 2007/097260, a method of enhancement of the production rate by a culture apparatus using a porous membrane is disclosed.
A polylactic acid can be produced by a method by ring-opening polymerization of lactide, which is a cyclic dimer of lactic acid, or a method by direct polymerization of a raw material lactic acid. In the lactide method, lactic acid is once oligomerized and then depolymerized while isolating lactide produced, which is then subjected to ring-opening polymerization in the presence of a catalyst. In this method, the polymerization process is complicated and, hence, requires much labor and cost. Since, in this process, impurities in the raw material lactic acid can be removed by the operation of lactide isolation, a raw material lactic acid of relatively low quality can be used. However, since impurities in the raw material lactic acid, such as inorganic ions, cause decrease in the yield of lactide to be isolated, the raw material lactic acid needs to be relatively free from impurities. On the other hand, in the direct polymerization method, the raw material lactic acid is subjected to direct dehydration polycondensation in the presence of a catalyst. In this method, simplification of the process can be expected compared to the lactide method, but impurities that inhibit the polymerization need to be preliminarily removed from the raw material lactic acid, to provide a high-quality raw material lactic acid. Thus, the purification efficiency of lactic acid influences the enhancement of productivity of lactide and polylactic acid.
Production of lactic acid by microbial fermentation is carried out while adding an alkaline substance to the culture medium to maintain the optimum pH for the microbial fermentation, and examples of the alkaline substance to be added to the culture medium include calcium hydroxide. In cases where calcium hydroxide was used, the lactic acid produced by microbial fermentation exists in the culture medium as calcium lactate. By adding an acidic substance (e.g., sulfuric acid) to the culture medium after completion of the culture, a solution of free lactic acid can be obtained, but a calcium salt (e.g., calcium sulfate) is by-produced as an impurity.
As a method for separating lactic acid by removing the by-produced calcium salt, in cases where an insoluble calcium salt such as calcium sulfate precipitates, a method by filtration through qualitative filter paper or the like is used, but a small amount of the calcium salt dissolved in the solution cannot be removed, and remains in the lactic acid-containing solution. Therefore, in cases where this filtrate containing lactic acid is, for example, concentrated in a later purification step, the calcium salt and other soluble inorganic acids deposit (precipitate) in the solution containing free lactic acid, which has been problematic. It is known that, if the lactic acid-containing solution from which inorganic ions have not been sufficiently removed is heated by an operation such as distillation, the inorganic ions allow racemization and oligomerization of lactic acid to proceed.
Examples of the method of removal of small amounts of inorganic ions from a lactic acid-containing solution include methods using ion-exchange resins (e.g., see Japanese Trans-lated PCT Patent Application Laid-Open No. 2001-506274). However, to maintain the ion-exchange performance of the ion-exchange resin, the ion-exchange resin must be regenerated periodically. Further, since regeneration of an ion-exchange resin is carried out by using large amounts of an aqueous sodium hydroxide solution and an aqueous hydrochloric acid solution, a large amount of waste fluid is discharged during the regeneration, so that a large amount of cost is required for waste liquid disposal, which has been problematic. Further, repeated regeneration of an ion-exchange resin results in decrease in the regeneration rate of the ion-exchange resin, as well as decrease in the ion-exchange performance, leading to decrease in the removal rate of inorganic acids, which have been problematic.
Further, methods of removal of small amounts of inorganic ion components such as calcium components from a lactic acid-containing solution using an electrodialyzer with a bipolar membrane are also known (e.g., see JP 2005-270025 A). However, the bipolar membrane used in these methods is expensive and the efficiency of removal of inorganic salts such as calcium salts is not necessarily high, which have been problematic.
Further, methods of removal of inorganic salts from a lactic acid-containing solution using a nanofiltration membrane have been disclosed (e.g., see U.S. Pat. Nos. 5,503,750, 5,681,728 and US 2004/0033573). However, a step of recovery of lactic acid by distillation, the effect of distillation on the yield of lactic acid, and the possibility of application of the obtained lactic acid to industrial-scale production of a polylactic acid by direct polymerization have not been disclosed.
Further, in JP 6-279577 A, JP 7-133344 A, JP 8-188642 A and JP 9-31170 A, the fact that the amounts of particular impurities need to be less than particular levels to obtain a high-molecular-weight polylactic acid has been disclosed, but the influences of impurities on the thermal stability, mechanical strength and hue, which are important factors for the processability of polylactic acids, have not been disclosed.
It could therefore be helpful to provide a method for producing lactic acid with high productivity, which lactic acid can be applied to industrial-scale production of a polylactic acid by direct polymerization and can be used for high-yield synthesis of lactide; and methods for producing lactide and a polylactic acid using the lactic acid. Further, it could be helpful to provide a polylactic acid having excellent thermal stability, mechanical strength and hue, and to provide lactic acid in which the amounts of specific impurities are not more than certain amount, and lactide and a polylactic acid obtained using the lactic acid as a raw material.